Porous sintered compact of titanium oxide for production of metallic titanium through direct electrolytic process and process for producing the same

ABSTRACT

A porous sintered compact of titanium oxide of the present invention is characterized in that it has a porosity of 20 to 65% and a hardness of 60 (HV) or higher, or characterized in that it has a porosity of 20 to 65%, a specific surface area of 0.1 to 5.0 m 2 /cm 3 , a volume ratio of pores with 0.3 to 100 μm diameter to be 10% or higher to the total pore volume and a hardness of 60 (HV) or higher. Using this porous sintered compact as an electrolytic raw material in the method in which titanium oxide is reduced by electrolysis with an electrolyte composed of a molten salt enables efficiently obtaining metallic titanium. The electrolytic process using a molten salt is attracting attention as a process capable of directly obtaining metallic titanium from titanium oxide with lower cost than in conventional processes, and the employment of the above porous sintered compact would promote its realization remarkably.

TECHNICAL FIELD

The present invention relates to a process for producing metallictitanium in which a titanium raw material of oxide form is reduced tometallic titanium in a molten salt by electrolysis, a sintered compactfor raw material electrode capable of efficiently obtaining metallictitanium, and a process for producing the same.

BACKGROUND ART

Metallic titanium is excellent in corrosion resistance and designproperty with proper elasticity, and extensively applied to aviationmaterials, roof materials, golf heads, materials for heat exchanger,chemical plants and the like as a material with a high strength obtainedin the same mass, so called high specific strength. In recent years, itsapplication is increasingly extended to medical equipment and the likeas a metal nontoxic to human body. However, since metallic titanium isexpensive as metal because many processes are required for smelting inits production, a further inexpensive industrial production method withhigh productivity is desired.

Metallic titanium is generally produced by chlorinating a titanium oxide(mainly TiO₂) of raw material to titanium tetrachloride followed byrefining by distillation, and reductively reacting it with Mg to formspongy metallic Ti. In another process, Na can be used also for thereduction step. The process using Mg is called the Kroll process, andthe process using Na the Hunter process.

Since it is dangerous to rapidly conduct this reductive reaction oftitanium tetrachloride that is an exothermic reaction, a long time isrequired for its sufficiently controlled reaction, and the productivityis significantly limited because of the batch system. Further, althoughMgCl₂ generated by the reduction is separated into Mg and Cl₂ throughmolten salt electrolytic process for re-using, about ⅔ of the power usedfor the smelting of metallic titanium is consumed in this molten saltelectrolysis. Accordingly, a process capable of reducing the productioncost by shortening the reaction time and effectively using the power isdemanded.

Recently, a direct electrolytic process for electrically reducing ametallic oxide in a chloride molten salt is proposed. This process isattracting attention as a process capable of significantly rationalizingthe production process, since titanium can be directly electrolyzed onceit can be applied to titanium.

This direct electrolytic process, which is disclosed in PublishedJapanese Translation of PCT International Publication No. 2002-517613,utilizes the phenomenon that, for example, when electric current isconducted into a molten salt with a cathode composed of metallictitanium containing oxygen, a migration reaction of the oxygen in thetitanium to the electrolyte proceeds more preferentially thanprecipitation of the metal ion in the molten salt, that is theelectrolyte, onto the surface of the titanium of the cathode.

It is described that, in addition to the oxygen contained in a conductorsuch as metallic titanium, oxygen in a titanium oxide can be similarlyremoved, if it is in contact with the cathode, to form metallictitanium. In the production of metallic titanium, TiO₂ is used in theform of a 40-50% porous body as the electrolytic raw material by makingits powder to slurry and molding it into various shapes by slip castingfollowed by sintering.

The molten salt electrolytic process is presumed to be capable ofeffectively removing the oxygen present in titanium that is a conductor.However, the reduction of oxide titanium to metallic titanium is no easytask, and there is a need for solving various problems.

DISCLOSURE OF THE INVENTION

The present invention has an object to provide a raw material sinteredcompact capable of enhancing the generation efficiency of titanium in aprocess wherein titanium oxide is reduced to metallic titanium by theelectrolysis with an electrolyte composed of a molten salt, a processfor producing the sintered compact, and a process for efficientlyproducing metallic titanium by use of the sintered compact.

The present inventors made various examinations for the process fordirectly obtaining metallic titanium by electrolyzing titanium oxide(TiO₂) of raw material in a molten salt such as CaCl₂, MgCl₂ or thelike. The sintered compact of titanium oxide as the raw material wasproduced by a general sintering method using the powder thereof.Particularly, as a result of reduction of the titanium oxide as acathode conductor or in contact with a cathode, metallic titanium couldbe obtained, but the generation efficiency of Ti was extremely poor, andthis process was found not to be applicable to industrial production asit is.

When a compound or the like is reduced to metal through an electrolyticprocess, the generated quantity of the metal is proportional to theprovided quantity of electricity according to the Faraday's law. In thepresent specification, the ratio of the actual quantity of metallictitanium obtained by electrolysis to an ideal generation quantity ofmetallic titanium according to the Faraday's law, which corresponds tothe provided quantity of electricity, is referred to as the generationefficiency of titanium.

Oxide titanium has fairly good electric conductivity at a hightemperature where the molten salt is used as the electrolyte. Therefore,the present inventors assumed at first that the reduction would beperformed according to the mechanism that, by carrying electric currentto the titanium oxide as a cathode or the titanium oxide in contact witha conductor cathode, the oxygen contained therein is ionized anddesorbed on the cathode surface to thereby form metallic titanium.

However, an attempt to explain the reductive reaction phenomenon causedon the basis of such a mechanism did not necessarily and sufficientlyillustrate the phenomenon. Further, improvements in the process orcondition based on the mechanism to improve the generation efficiencyhardly exhibited the effect.

For example, if the reductive reaction proceeds by the ionization ofoxygen as described above, the generation quantity of metallic titaniumshould be increased as the electric current is increased. However, evenif the current is increased, the generation quantity of titanium is notproportionally increased. Further, the titanium oxide has the propertyof increasing electric conductivity at high temperature, but does notpass the current so much as metal, and there is a limitation insufficiently increasing the current. Further, the generation quantity oftitanium to the same current value is satisfactory just after startingthe electrolysis, but largely deteriorated with the lapse of time.

In the course of such an examination, it was found that there is thephenomenon that the generation efficiency of titanium is greatlyimproved when titanium oxide laid in a porous state due to imperfectsintering is used as the raw material or as an electrode as for the rawmaterial. Further, it was also found that the electric energization ofthe porous body is not always required, and metallic titanium isgenerated once the porous body is in a position as close as possible toa conductor used as the cathode, even if not surely in contacttherewith.

It was assumed from this that the reductive reaction of titanium oxidemight include, in addition to electric ionization of oxygen, generationof metallic titanium resulted from that Ca generated by electrolysis ofCaCl₂ or the like used as the electrolyte owing to the electricenergization reduces the titanium oxide. Ca is an extremely activemetal, which reacts, even if generated by electric energizing, withoxygen or dissociated chlorine in the electrolyte, or oxygen or nitrogenin the atmosphere to form another compound, and extinguishes. However,when the titanium oxide is the cathode itself, or present just close tothe cathode, Ca would reduce it prior to extinguishing, and generatemetallic titanium.

When the experimental result of the metallic titanium generation thusexamined is considered from the standpoint that this reductive reactioncaused by the Ca generated by this electrolysis is also included, manyaspects can be rationally explained. The significant improvement ingeneration efficiency of titanium by using a porous sintered compact asthe raw material is also considered to be attributable to that thesurface area to the same mass or the specific surface area is increaseddue to adaptation of the porous sintered compact, thus increasing thearea to make contact with the Ca which is generated by electrolysis anddispersed to the molten salt.

When metallic titanium is produced by electrolysis, the generationefficiency that how much the generation quantity can get close to thequantity estimated from the Faraday's law, and the generation rate,depending on the provided quantity of electricity, are important.

There is an occasion that, even if the generation rate is high with alarge electric current to the same potential in the initial stage ofelectrolysis, the current may become difficult to pass in accordancewith the continuation of electrolysis, thus blocking the electrolysis.In spite of a high generation rate in the initial stage of electrolysis,the sintered electrode may occasionally be collapsed, disenabling theelectrolysis.

As a result of examinations for raising the porosity to increase thespecific surface area while variously changing the processes forproducing the porous sintered compact, two serious problems becameclear. One problem is that the generation efficiency or generation ratecannot be greatly increased only by raising the porosity, and the otheris that continuation of electrolysis for obtaining metallic titaniumwith a sufficiently low oxygen content may cause collapse of the poroussintered compact, disenabling further reduction.

The porosity is calculated as a shortage of the apparent densitydetermined from measurement of the weight and volume of the sinteredcompact to the theoretic density (4.2 g/cm³) of compact TiO₂ solid.However, since it could not be determined whether or not the poroussintered compact is suitable for molten salt electrolysis only by themagnitude of porosity, the total surface area per apparent unit volumeor specific surface area by gas adsorption process (BET process) and thepore distribution by mercury porosimetry were further measured incombination as the evaluation of the surface to make contact with themolten salt.

It is assumed that the surface area contactable with the molten salt ofthe porous sintered compact can be measured by the BET process, and thedistribution of pore diameters which the molten salt can be crawled incan be known by the mercury porosimetry.

The specific surface area and pore distribution were measured for someporous sintered compacts, and these values were collated with thegeneration efficiency and generation rate of metallic Ti in molten saltelectrolysis. As a result, it was found that excellent efficiency andrate can be obtained when these measurement values are within specifiedranges. The specific surface area and pore distribution do notnecessarily correspond with the magnitude of porosity.

The larger the specific surface area is, the more the area to makecontact with the molten salt or Ca in the molten salt increases.However, the presence of the upper limit is attributable to that, whenthe area becomes excessively large, the pore diameter becomes too smallto discharge the resultant CaO.

It was also found from the measurement of pore distribution that, if thenumber of pores of diameters within a specified range is not less than acertain value, the drop of the generation efficiency during the progressof electrolysis can be mitigated and maintenance of the generation ratecan be secured.

The reason for causing such a phenomenon was not necessarily clarified.However, if the Ca generated by electrolysis significantly affects thereduction of titanium oxide, an extremely small pore diameter disturbsthe reaction since a reduction product cannot be easily removed from thereactive surface, resulting in the deterioration of generationefficiency due to the suspension of the reaction, and an excessivelylarge pore diameter also arrests the progress of the reaction since thegenerated Ca cannot stay around. Accordingly, the presence of furthermore pores of proper sizes may be important for preventing thedeterioration of generation efficiency.

Based on such a result, production conditions for obtaining a poroussintered compact, the specific surface area and pore distribution to bewithin optimum ranges were examined. In the method of making the powderto slurry by addition of water and molding by slip casting followed bysintering, it is easy to enhance the porosity, but it is difficult tocontrol the specific surface area or pore distribution. Further, thismethod is not preferable since the sintered compact may occasionallycollapse with the progress of electrolysis.

In contrast to this, it was confirmed that a porous sintered compacthaving a necessary specific surface area or pore distribution can beobtained by controlling the grain size of powder, performingpress-molding by use of dies with controlled pressurizing force, andcontrolling the temperature and time of sintering.

As described above, it was found that the generation efficiency andgeneration rate of metallic titanium can be improved by limiting, in theporous sintered compact used as the electrolytic raw material, not onlythe porosity but also the specific surface area as well as the poredistribution. However, when the electrolysis is continued to obtainmetallic titanium with a sufficiently low oxygen content, the poroussintered compact is frequently collapsed, thus disabling the furtherreduction.

The reason for this is assumed that the porous sintered compact havingan intended specific surface area or pore distribution is frequentlylaid in an imperfectly sintered state because it can be more easilyobtained at a lower sintering temperature, and this causes the collapse.

As a result of examinations for a compact easy to collapse and a compactcausing no collapse, it was confirmed that no collapse is caused with ahardness of 60 HV or higher after sintering even if the electrolysis iscontinued until oxygen is sufficiently reduced. When the porous sinteredcompact has an intended porosity with a hardness of said value or higherafter sintering, its collapse can be inhibited during electrolyticreduction regardless of the specific surface area or pore distribution.

It was assumed that the sintering is required to progress at a furtherlow temperature in order to ensure a high hardness after sintering withthe intended specific surface area and pore distribution of porousstate. As a result of further examinations for the production conditionof such a sintered compact, it was found that addition of a small amountof titanium suboxide such as TiO, Ti₂O₃, Ti₃O₅ or the like issufficient.

This is considered to be attributable to that by adding the titaniumsuboxide to the raw material of titanium oxide powder, the sintering incontacts between grains is promoted, even if the density of the compactbefore heating is not high, to cause the compact in a sufficientlysintered state as it is porous.

When the porous sintered compact thus-obtained is electrolyzed in astate of being disposed as close as possible to an electric conductorthat is a cathode, the reductive reaction proceeds even if it is notnecessarily in contact with the cathode conductor to pass the current.However, when the electrolysis is executed by use of a cathode composedof an integrated electrode in which the porous sintered compact isclosely packed around a core of a good-electric-conductor, thegeneration efficiency of titanium can be further greatly improved.

The respective marginal conditions were confirmed based on theabove-mentioned knowledge to complete the present invention. The gist ofthe prevent invention is as follows.

(1) A porous sintered compact of titanium oxide for production ofmetallic titanium through direct electrolytic process, in which it has aporosity of 20 to 65% and a hardness of 60 (HV) or higher.

(2) A porous sintered compact of titanium oxide for production ofmetallic titanium through direct electrolytic process, in which it has aporosity of 20 to 65%, a specific surface area of 0.1 to 5.0 m²/cm³, anda volume ratio of pores with 0.3 to 100 μm diameter to be 10% or higherto the total pore volume.

(3) A porous sintered compact of titanium oxide for production ofmetallic titanium through direct electrolytic process, in which it has aporosity of 20 to 65%, a hardness of 60 (HV) or higher, a specificsurface area of 0.1 to 5.0 m²/cm³, and a volume ratio of pores with 0.3to 100 μm diameter to be 10% or higher to the total pore volume.

(4) A process for producing a porous sintered compact of titanium oxideaccording to any one of (1) to (3), comprising using a titanium oxidepowder having a grain size of 0.2 to 2000 μm, molding it into a requiredshape with pressurization in a range of 9.8 to 78.5 MPa, and sinteringat 1100 to 1500° C. for 0.5 to 10 hours.

(5) A process for producing a porous sintered compact of titanium oxideaccording to any one of (1) to (3), comprising adding and mixing 0.1 to40%, based on mass, of a titanium suboxide to a titanium oxide powderfollowed by molding into a required shape, and sintering at 900 to 1400°C. for 0.5 to 10 hours.

(6) A process for producing a porous sintered compact of titanium oxideaccording to any one of (1) to (3), comprising using a titanium oxidepowder having a grain size of 0.2 to 2000 μm, adding and mixing 0.1 to40%, based on mass, of a titanium suboxide powder thereto followed bymolding into a required shape with pressurization in a range of 9.8 to78.5 MPa, and sintering at 900 to 1400° C. for 0.5 to 10 hours.

(7) A process for producing metallic titanium, comprising using a poroussintered compact of titanium oxide according to any one of (1) to (3),arranging it adjacently to a conductor or closely adhered around theconductor to constitute a cathode, dipping it in a molten saltelectrolyte of 800 to 1050° C. containing 40 mass % or more of calciumchloride, and reducing it by electric energization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing an apparatus for producingmetallic titanium through molten salt electrolytic process; and

FIG. 2 are illustrative views of a structure of a cathode constituted bya workpiece material, wherein (A) shows an electrode in which smallmasses of a porous sintered compact of titanium oxide are arrangedadjacently to the circumference of a metallic conductor, and (B) showsan electrode in which the workpiece material of the porous sinteredcompact of titanium oxide is closely adhered around the metallicconductor as being a core.

BEST MODE FOR CARRYING OUT THE INVENTION

A porous sintered compact of titanium oxide according to the presentinvention is, as an example, placed in the vicinity of a cathode or asan integrated electrode in an electrolytic cell containing anelectrolyte composed of a molten salt, as schematically shown in FIG. 1,and reduced to metallic titanium. In FIG. 1, an anode 3 and a cathode 4are dipped in a molten salt cell 2 retained in a container 1 which canbe heated and corrosion-resistant to molten salts, and direct current issupplied from a power supply 5 to perform electrolysis.

In this case, the cathode may be constituted, for example, into (A) anelectrode in which a wire basket 7 allowing the circulation of aliquefied molten salt is placed around a metallic conductor 6, and smallmasses of a porous sintered compact of titanium oxide 8 are placedadjacently to the conductor 6 in the wire basket 7, as schematicallyshown in FIG. 2, or (B) an electrode in which the raw material of theporous sintered compact of titanium oxide is adhered closely around themetallic conductor 6 as being a core. These electrodes may have anyshape such as bar-like, sheet-like or other shape.

The porous sintered compact of titanium oxide which is reduced tometallic titanium can be a porous body having a porosity of 20 to 65%and a hardness of 60 HV or higher.

The reason for setting the porosity to 20% or higher is that a porositybelow 20% causes a significant deterioration of generation efficiency ofTi. This is attributable to that pores spatially isolated and blocked tothe outside are increased, resulting in a relative decrease in thecontact area with the molten electrolyte. On the other hand, with aporosity exceeding 65%, not only the shape of the raw material startscollapsing in the middle of the reduction step, disabling a sufficientreduction step, but also the recovery of metallic titanium becomesdifficult.

To increase the porosity, an addition of a large quantity of a binder orthe like is required at the time of producing a sintering material bypressing. However, since such a binder or the like must be eliminated byheating at the time of sintering, which deteriorates the productivity,the porosity is desirably set to 20% or higher and lower than 40%.

The reason for setting the hardness of the porous sintered compact to 60HV or higher is that the shape of the raw material may be collapsedduring electrolytic reduction due to insufficient sintering when thehardness is below 60 HV. In case of a porous body having an intendedporosity, the upper limit of hardness is not particularly specified.

The porous sintered compact of titanium oxide as an electrolytic rawmaterial which is reduced to metallic titanium can have a porosity of 20to 65%, a specific surface area of 0.1 to 5.0 m²/cm³, and a volume ratioof pores with 0.3 to 100 μm diameter to be 10% or higher to the totalpore volume.

The reason for setting the porosity to 20 to 65% is the same asdescribed above. Similarly, to obtain an increased porosity, an additionof a large quantity of a binder or the like is required at the time ofproducing a sintering material by pressing. Since the elimination ofsuch a binder or the like by heating must be performed at the time ofsintering, which leads to deterioration of productivity, the porosity isdesirably set to 20% or higher and lower than 40%.

The reason for setting the specific surface area to 0.1 to 5.0 m²/cm³ isthat the specific surface area either smaller than 0.1 m²/cm³ or largerthan 5.0 m²/cm³ causes a deterioration of generation efficiency with adecreased generation rate. The specific surface area is measuredaccording to a method called the BET process for determining from themonomolecular layer adsorption based on BET adsorption isothermalprocess of inert gas such as argon, nitrogen or the like.

The reason for setting such an optimum range for the specific surfacearea is that the area smaller than 0.1 m²/cm³ inhibits the reductivereaction because the area to make contact with the molten salt is toosmall, and with the area larger than 5.0 m²/cm³ resulting in smallerpore diameter deteriorates the circulation of the molten salt to disturbthe rapid elimination of the reaction product, which may alsoconsequently inhibit the reductive reaction.

The pore distribution is determined by mercury porosimetry. In themercury porosimetry, the diameter of pores and the volume of the poreshaving the diameter thereof can be measured, and the total pore volumecan be determined from the integration of pore distribution curves. Thevolume ratio of pores having diameters ranging from 0.3 to 100 μm is setto 10% or higher to the total pore volume.

The reason is that the volume ratio of pores having diameters smallerthan 0.3 μm or larger than 100 μm hardly affects the generationefficiency, but a volume ratio below 10%, for pores having diametersranging from 0.3 to 100 μm, results in a remarkable decrease in thegeneration efficiency. To keep the generation efficiency, at least 10%or more of the volume ratio is required for the pores ranging from 0.3to 100 μm. The higher the volume ratio is, the more the generationefficiency is improved. Therefore, the volume ratio is desirably closerto 100%.

Further, the porous sintered compact of titanium oxide of theelectrolytic raw material which is reduced to metallic titaniumdesirably has a hardness of 60 or higher by Vickers hardness (HV), inaddition to a porosity of 20 to 65%, a specific surface area of 0.1 to5.0 m²/cm³, and a volume ratio of pores with 0.3 to 100 μm diameter tobe 10% or higher to the total pore volume.

The reason is that a hardness lower than 60 HV may result in the shapecollapse of the workpiece material during electrolytic reduction becauseof the insufficient sintering as described above. Although the poroussintered compact of titanium oxide for electrolytic reduction is likelyto be insufficiently sintered in order to obtain a high porosity of 20to 65%, the shape collapse during electrolysis is hardly caused if it issintered so as to ensure a hardness of 60 HV or higher. In this case,the upper limit of hardness is not particularly imposed when theporosity is within the above range.

The inhibition of the shape collapse by setting the hardness to 60 HV orhigher is effective regardless of the specific surface area or the poredistribution. Accordingly, when the hardness is set to 60 HV or higherin a porous sintered compact of oxide titanium having a porosity of 20to 65%, a specific surface area of 0.1 to 5.0 m²/cm³, and a volume ratioof pores with 0.3 to 100 μm diameter to be 10% or higher to the totalpore volume, the shape collapse during electrolysis can be inhibited,and an extremely excellent raw material for electrolytic reduction canbe obtained.

As the raw material of the porous sintered compact, oxide titaniumpowders such as rutile, anatase and the like are used. Impuritiesincluded in the raw material are frequently taken into metallic titaniumas they are, although some of them are eliminated during electrolyticreduction. Accordingly, a material with impurities as less as possibleis preferably used.

The average grain size of the raw material powder is set to the rangefrom 0.2 to 2000 μm. This is because if a large amount of grains smalleror larger than this range is included, it may be difficult to maintainthe molded shape at the time of pressure-molding the mixed powder. Thisis also because the collapse is likely to occur during electrolysisbecause of an insufficient strength of the sintered compact aftersintering, or an intended porosity may not be obtained.

A binder or the like can be added and kneaded to the powder of the rawmaterial, particularly, when a higher porosity is required, or it isdifficult to maintain the shape after pressure molding, but it may notbe added. The raw material is molded into a desired shape by use of dieswith pressurization in the range of 9.8 to 78.5 MPa. A pressurizingforce lower than 9.8 MPa might make it difficult to maintain the shapeafter taken out from the dies, and pressurization higher than 78.5 MPamight make it impossible to obtain a specific surface area or porediameter distribution within an intended range after sintering.

The shape of the porous sintered compact is not particularly limited.For example, in case of the electrode in which the small massive poroussintered compact 8 is retained by use of the basket 7 as shown in FIG.2(A), excessively small masses, which may be fallen through the meshesof the basket, are difficult to handle, and excessively large massesneed a long time for reduction step, resulting in the deterioration ofthe generation rate. Accordingly, the compact is preferably made tomasses with a maximum diameter of about 2 to 30 mm. The small masses mayhave any shape such as spherical, columnar, cuboid, or other shapewithout having any particular limitation.

In the production of the small massive porous sintered compact, althoughthe compact before sintering may have the above massive shape, thecompact can be sintered as a larger sheet-like, bar-like, cylindrical orcuboid, or the like, and then pulverized to small masses of the abovesize.

In case of the electrode in which the material is closely adhered arounda metallic conductor as being a core, as shown in FIG. 2(B), themetallic conductor and the electrode can have any shape such as bar,plate or other shape without having any particular limitation. However,the distance from the conductor to the surface of the porous materialwhich directly makes contact with the molten salt electrolyte isdesirably set to 30 mm or less. The reason is that a distance exceeding30 mm makes it difficult to increase the current density, despite goodelectric conductivity.

When the electrode of such an integrated structure is used, theelectrode is constituted by molding the raw material powder kneaded bodyas the workpiece material into an electrode shape with the metallicconductor followed by integration by virtue of simultaneous sintering,or mechanically closely adhering the metallic conductor to the poroussintered compact. As the metal of the conductor to be the core,stainless steel or iron may be used, but metallic titanium is preferablyused from the point of inclusion of impurities.

Although the electrode which is the electrolytic workpiece material mayhave either structure of FIG. 2(A) or (B), the integrated type (B) usingthe conductor as the core is desirable in the practical production fromthe point of satisfactory workability of the electrode such as handlingin electrolysis, high generation efficiency, and the like.

The raw material powder is pressure-molded, sufficiently dried ifnecessary, and sintered at 1100 to 1500° C. for 0.5 to 10 hours. Whenthe temperature is lower than 1100° C., or the sintering time is lessthan 0.5 hour, the porous sintered compact cannot have a sufficienthardness because of insufficient sintering.

A sintering temperature exceeding 1500° C. or a heating time exceeding10 hours may result in a porosity of smaller than 20%, a specificsurface area below 0.1 m²/cm³, a volume rate of pores of 0.3 to 100 μmdiameter to be below 10% to the total pore volume.

In the process for producing the porous sintered compact, when the rawmaterial powder is pressure-molded after adding and mixing 0.1 to 40%,based on mass, of a titanium suboxide powder such as TiO, Ti₂O₃, Ti₃O₅or the like, and then sintered, sintering sufficiently proceeds even ina sintering temperature range as low as 900 to 1400° C., and a hardnessafter sintering of 60 HV or higher can be ensured. The titanium suboxideis a titanium oxide deficient in oxygen to titanium oxide TiO₂, whichmay have any composition, and can be added alone or in mixture.

When 0.1 to 40% of the powder of the titanium suboxide with an averagegrain size of 0.2 to 2000 μm is added and mixed thereto, similarly tothe powder of the titanium oxide, and then molded into a desired shapewith pressurization in the range of 9.8 to 78.5 MPa as described above,a porous sintered compact of titanium oxide having a porosity of 20 to65%, a specific surface area of 0.1 to 5.0 m²/cm³, and a volume ratio ofpores with 0.3 to 100 μm diameter to be 10% or higher to the total porevolume, and a hardness of 60 HV or higher can be easily obtained bysintering at 900 to 1400° C. for 0.5 to 10 hours.

The thus-produced porous sintered compact of titanium oxide is filled,in case of small masses, in the basket 7 surrounding the conductor 6, asshown in FIG. 2(A), to form the electrode. The conductor 6 may be a goodelectric conductor such as titanium, stainless steel, iron or the like,and the basket 7 may be formed of stainless steel or ceramics excellentin corrosion resistance since conductivity is not particularly required.

Since the reductive reaction is more difficult to occur as the distancebetween the porous sintered compact and the electric conductor islarger, an inner surface of the basket 7 is desirably set within 50 mmfrom the surface of the conductor 6. In case of the porous sinteredcompact of the electrode shape as shown in FIG. 2(B), in which theporous sintered compact is integrally molded around the conductor asbeing a core, it can be applied to the electrolysis as it is.

When an electrolytic cell of the structure shown in FIG. 1 is used, theprocess for producing metallic titanium through electrolytic reductionby use of the cathode composed of the porous sintered compact oftitanium oxide as above is as follows.

In order to facilitate the promotion of the electrolytic reductionprocess, any molten salt which satisfies the following conditions can beused as the electrolyte 2 without particularly limiting otherconditions.

(A) The salt or an oxide of its metal ion, even if adhered or penetratedinto the porous sintered compact after the end of reduction, can beeasily washed away with water or a weak acid.

(B) The metal generated by electrolysis of the electrolyte itself canreduce the titanium oxide.

(C) The salt can be laid in a molten state at a temperature of not lessthan the melting point of the metal generated in B and not greater thanthe melting point of Ti.

As molten salts satisfying these conditions, CaCl₂ may be used alone,otherwise MgCl₂, BaCl₂, NaCl, CaF, MgF or the like may be added toCaCl₂, which is a main component, i.e. makes up 40 mass % or more, forthe purpose of decreasing the melting point or adjusting the viscosityor the like. When CaCl₂ is below 40 mass %, it may be difficult toeliminate the molten salt or oxide adhered to the porous sinteredcompact after reduction.

As the anode 3, although any conductor can be used without particularlimitation, graphite, stainless steel, iron or the like may be used. Thetemperature of the molten salt during electrolysis is desirably set to800 to 1050° C. A temperature lower than 800° C. may result indeterioration of the fluidity of the molten salt, which inhibits theprogress of electrolysis. Since the melting point of Ca which is assumedto be generated by electrolysis is 843° C., the progress of thereduction reaction related to Ca is delayed. Therefore, an excessivelylow temperature is not desirable.

A temperature higher than 1050° C. should be avoided since it results innot only waste of heating energy but also excessive evaporation of themolten salt, and further may promote oxidation of the reduced titanium.In order to avoid the wasteful consumption of the Ca generated, theatmosphere in the container is desirably filled with inert gas duringelectrolysis.

EXAMPLE 1

Using a titanium dioxide of rutile type (99% or more), anatase type (99%or more), or rutile type with slightly poor purity (95% or more) whichhas a different grain size range of powder as a raw material andcommercially available TiO as a titanium suboxide, these were mixedtogether, and then pressure-molded by use of dies into disks of 25 mm indiameter and 10 mm in height. The molded disks were sintered in theatmosphere with varied holding temperatures and varied holding times,and the porosity and hardness after sintering were measured.

The production conditions of the sintered compacts and the measurementresults of porosity and hardness after sintering are shown in Table 1.As is apparent from the results, a porous sintered compact having aporosity and a hardness within an intended range can be obtained byadjusting the grain size range, the quantity of titanium suboxide, andthe sintering temperature and time. TABLE 1 Addition Sintered Grain SizeAmount Molding Condition Test Raw Range of Tio Pressure Temperature TimePorosity Hardness No. Material (μm) (%) (MPa) (° C.) (hr) (vol. %) (HV)Remarks A01 Rutile 0.2-0.9 *0 15 900 11 45 *55 Comparative Example A02 ″0.5 20 900 1 45 62 Inventive Emample A03 ″ 1.0 80 1000 3 35 100Inventive Emample A04 ″ 1.0 20 1100 4 25 150 Inventive Emample A05 ″ *0120 1200 6 *5 320 Comparative Example A06 ″ *0 20 1300 5 *2 850Comparative Example A07 ″ *0 20 1400 5 *1 1050 Comparative Example A08Anatase 0.1-0.5 0.5 20 900 4 65 70 Inventive Example A09 ″ 0.5 20 900 155 80 Inventive Example A10 ″ 1.0 20 1000 3 50 120 Inventive Example A11″ 1.0 20 1100 4 45 150 Inventive Example A12 ″ 1.0 100 1200 6 40 130Inventive Example A13 ″ 2.0 10 1300 5 40 120 Inventive Example A14 ″ 5.020 1400 5 35 130 Inventive Example A15 95% 150-250 *0 20 900 1 40 *54Comparative Rutile Example A16 ″ 0.5 50 900 1 42 75 Inventive ExampleA17 ″ 1.0 20 1000 3 35 120 Inventive Example A18 ″ 1.0 20 1100 4 33 200Inventive Example A19 ″ 1.0 80 1200 6 34 250 Inventive Example*denotes a value out of the range defined by the present invention.

EXAMPLE 2

With the raw material powders and the sintering conditions shown inTable 1, cuboids of 10×20×10 mm (width, length, height) werepressure-molded by use of dies. At the same time, a titanium bar 2 mm indiameter and 30 mm in length was stuck to its longitudinally midpoint ofeach cuboid to a depth of 15 mm to form a conductor for electricenergization, and then integrally sintered to produce sintered rawmaterials.

Using CaCl₂ alone or in combination with NaCl, MgCl₂, CaF₂ or the likeas the molten salt, a graphite electrode as the anode, and a conductivetitanium bar as a supporting and electric energizing terminal, the lowerhalf portion of 10×10×10 mm of each cuboid material was dipped in theheated molten salt, and electrolyzed.

The porosity and hardness of the workpiece materials, the composition ofelectrolytic cell, the cell temperature, the current density, theelectric energizing time, the shpe of electrode, the generationefficiency of titanium, and the like are collectively shown in Table 2.The generation efficiency of titanium is shown as the ratio of theactual generation quantity to the Ti quantity calculated, on assumptionthat TiO₂ changes Ti, by the Faraday's law from the current and timeemployed. The sintering conditions of the workpiece materials areidentical with those of the same test numbers shown in Table 1.

As is apparent from the results shown in Table 2, when the porosity iswithin the range determined by the present invention, the generationefficiency of metallic titanium is 20% or more, while the generationefficiency is poor with a low current density when the porosity is low.

The hardness after sintering provides an indication of whether thesintering is sufficiently performed or not. When the hardness is low,electrode collapse occurs even if the porosity is within the range ofthe present invention, and metallic titanium cannot be sufficientlyobtained. TABLE 2 Workpiece Material Composition of ElectrolyticCondition Generation for Sintering Molten Salt Cell Cell Current ※Efficiency of Sintering Porosity Hardness (mass %) Temperature DensityTime Shape of Titanium Test No. Condition (vol. %) (Hv) CaCl₂ NaCl MgCl₂CaF₂ (° C.) (A/cm2) (hr) Electrode (%) Remarks B01 A01 45 *55 100 0 0 0850 0.50 5.0 X 5 Comparative Example B02 A01 45 *55 50 20 20 0 900 0.8011.0 X 1 Comparative Example B03 A02 45 62 100 0 0 0 850 0.50 1.5 ◯ 85Inventive Example B04 A02 45 62 50 20 20 0 850 0.50 1.5 ◯ 85 InventiveExample B05 A02 45 62 40 30 30 10 850 0.50 1.5 ◯ 85 Inventive ExampleB06 A03 35 100 100 0 0 0 850 0.50 1.5 ◯ 80 Inventive Example B07 A04 20150 100 0 0 0 850 0.20 5.0 ◯ 75 Inventive Example B08 A05 *5 320 100 0 00 850 0.10 11.0 ◯ 5 Comparative Example B09 A06 *2 850 100 0 0 0 8501.00 0.8 ◯ 7 Comparative Example B10 A07 *1 1050 100 0 0 0 850 0.20 5.0◯ 2 Comparative Example B11 A08 65 *55 50 0 0 0 850 0.50 10.0 X 5Comparative Example B12 A08 65 *55 40 20 20 0 980 0.50 10.0 X 7Comparative Example B13 A08 65 *5 100 30 30 10 700 0.50 10.0 X 2Comparative Example B14 A09 55 80 100 0 0 0 850 0.80 1.0 ◯ 75 InventiveExample B15 A10 50 120 100 0 0 0 850 0.50 1.5 ◯ 85 Inventive Example B16A11 45 150 100 0 0 0 850 1.50 0.5 ◯ 70 Inventive Example B17 A12 40 130100 0 0 0 850 0.40 1.8 ◯ 70 Inventive Example B18 A15 40 *54 100 0 0 0850 0.50 1.5 X 5 Comparative Example B19 A16 42 75 100 0 0 0 850 0.501.5 ◯ 75 Inventive Example B20 A19 34 250 100 0 0 0 850 0.50 1.5 ◯ 70Inventive Example*denotes a value out of the range defined by the present invention.※ Evaluation of electrode state◯: Maintaining the original shapeX: Collapsed

EXAMPLE 3

Using a titanium oxide powder including 95% or more of TiO₂ as the rawmaterial, disks of 25 mm in diameter and 10 mm in height were molded byuse of dies with varied pressurizing forces, the molded disks weresintered in the open air or in an argon atmosphere with variedtemperatures and times. For the resultant porous sintered compacts, theporosity, specific surface area, pore diameter distribution, hardnessand the like were measured.

The porosity was represented by the ratio obtained by determining theapparent density from the weight and dimension of each sintered compactand dividing the difference with the theoretical density of TiO₂ by thetheoretical density, and the specific surface area was determined by theBET process by using nitrogen as adsorption gas. The pore diameterdistribution was measured by use of a measuring device by mercuryporosimetry (manufactured by SHIMAZU, MICROMERITICS AUTOPORE 9200). Theproduction conditions and measurement results of these porous sinteredcompacts are collectively shown in Table 3.

Cuboids of 10×20×10 mm (width, length, height) were pressure-molded byuse of dies in the same condition as the above disks. At the same time,a titanium bar 2 mm in diameter and 30 mm in length was stuck to thelongitudinally midpoint position with square cross-section of eachcuboid to a depth of 15 mm to form a conductor for electric energizing,and then integrally sintered to form porous sintered compact electrodes.

The produced electrodes were electrolyzed for 10 hours by using CaCl₂alone or in combination of 10 mass % of NaCl as the molten salt andgraphite as the anode with a cell temperature of 900° C. and anelectrolytic potential of 3.0V. After the electrolysis, each electrodeshape was observed, the quantity of metal Ti generated on the electrodewas analyzed, and the ratio of the actual Ti quantity to the Ti quantitywhich was calculated from the current and time employed, on theassumption that TiO₂ changes Ti by the Faraday's law, was determined asthe generation efficiency.

The average rate of Ti generation per unit surface area of the electrodeand time was also determined. These results are collectively shown inTable 4. TABLE 3 Average Grain Size of Titanium Specific Pore VolumeRatio (%) Sintered Dioxide Raw Molding Sintering Surface Smaller LargerCompact Material Pressure Temperature Time Porosity Area than 0.3-100than Hardness No. (μm) (MPa) Atmosphere (° C.) (hr) (%) (m²/cm³) 0.3 μmμm 100 μm (HV) Remarks 1 0.3 *4.9 Open Air *850 0.5 *70  *6.00  85.014.4 0.6 50 Comparative Example 2 0.3 9.8 Open Air  900 0.5 60 4.50 92.2*7.7 0.1 55 Comparative Example 3 0.6 *98 Open Air 1200 2.0 *10  0.3739.4 56.2 4.4 400 Comparative Example 4 1500 *118 Open Air 1200 5.0 20*0.08  1.5 12.0 86.5 200 Comparative Example 5 750 *118 Open Air 12005.0 20 0.11 2.1 *9.2 88.7 180 Comparative Example 6 0.3 9.8 Open Air1000 2.0 64 5.00 80.0 19.4 0.6 70 Inventive Example 7 0.3 49 Open Air1200 4.0 43 2.20 0.6 98.9 0.5 120 Inventive Example 8 0.5 29 Open Air1300 10.0 27 1.38 87.5 11.1 1.4 220 Inventive Example 9 0.5 78 Open Air1200 5.0 20 0.73 8.3 85.7 6.0 250 Inventive Example 10 0.6 49 Open Air1200 2.0 30 1.10 8.1 87.5 4.4 210 Inventive Example 11 75 18 Open Air1100 2.0 35 0.50 20.5 77.4 2.1 180 Inventive Example 12 150 49 Open Air1200 4.0 35 0.30 0.2 95.7 4.1 180 Inventive Example 13 150 18 Open Air1100 7.0 45 0.39 0.2 87.3 12.5 120 Inventive Example 14 830 59 Ar 13004.0 28 0.22 0.2 43.1 56.7 220 Inventive Example 15 1500 78 Ar 1400 4.025 0.10 1.2 11.1 87.7 280 Inventive Example 16 1500 18 Ar 1400 2.0 280.15 1.1 19.0 79.9 180 Inventive Example*denotes a value out of the range defined by the present invention.

TABLE 4 Electrolytic Composition of Condition Generation of Titanium ※Sintered Molten Salt Cell Cell Current Generation Shape Test Compact(mass %) Temperature Density Efifciency Average Rate of No. No. CaCl₂NaCl (° C.) (A/cm²) (%) (kg/[hm²] Electrode Remarks C01 *1 100 0 900 2.566 — X Comparative Example C02 *2 100 0 900 2.5 5.5 0.74 ◯ ComparativeExample C03 *3 100 0 900 0 — — ◯ Comparative Example C04 *4 60 40 9000.3 45 0.73 ◯ Comparative Example C05 *5 60 40 900 0.8 25 1.08 ◯Comparative Example C06 6 60 40 900 1.5 68 5.52 ◯ Inventive Example C077 100 0 900 1.5 75 6.09 ◯ Inventive Example C08 8 100 0 900 2.0 60 6.49◯ Inventive Example C09 9 100 0 900 2.0 78 8.44 ◯ Inventive Example C1010 60 40 900 2.2 68 8.09 ◯ Inventive Example C11 11 80 20 900 1.5 655.27 ◯ Inventive Example C12 12 80 20 900 1.8 88 8.57 ◯ InventiveExample C13 13 80 20 900 1.2 80 5.19 ◯ Inventive Example C14 14 80 20900 0.8 75 3.25 ◯ Inventive Example C15 15 80 20 900 0.8 78 3.38 ◯Inventive Example C16 16 60 40 900 1.0 65 3.52 ◯ Inventive Example*denotes a value out of the range defined by the present invention.※ Evaluation of electrode shape◯: Maintaining the original shapeX: Collapsed

The following is found from the results of Tables 3 and 4. Namely, inTest No. C01 with excessively high porosity and specific surface areaand a low hardness, the electrode collapse occurred during electrolysis,and the electrolysis was thus cancelled. In Test No. C02 including poresof an excessively large number of small diameters in spite of a highporosity, the generation efficiency is poor.

In C03, the electrolytic current could be hardly conducted because ofextremely low porosity. In C04, the current density could not be raisedbecause of an excessively small specific surface area, and the averagegeneration rate was low. In C05 with small specific surface area and lowvolume ratio of preferable pores, both the current density and thegeneration efficiency are low.

Sintered compacts of Test Nos. C06-C16 are excellent in generationefficiency and average generation rate of titanium, and suitable forproduction of metallic titanium through direct electrolytic process.

Not only the porosity but also the specific surface area as well as thepore distribution significantly have influences on the generationefficiency and generation rate, and it is apparent that they must bewithin optimum ranges specified by the present invention.

The porosity, specific surface area and pore diameter distribution ofthe porous sintered compacts significantly depend on conditions in theproduction of the sintered compacts such as pressurizing force in powdermolding and sintering. When these conditions are set within the rangesspecified by the present invention, a satisfactory result can beobtained.

EXAMPLE 4

Using a titanium oxide powder including 95% or more of TiO₂ as the rawmaterial, TiO was mixed thereto as titanium suboxide, and disks of 25 mmin diameter and 10 mm in height were pressure-molded in the same manneras Example 3 followed by sintering. For the resultant porous sinteredcompacts, the porosity, specific surface area, pore diameterdistribution, hardness and the like were measured.

Similarly to Example 3, cuboids of 10×20×10 mm (width, length, height)were pressure-molded by use of dies in the same condition as the abovedisks. At the same time, a titanium bar 2 mm in diameter and 30 mm inlength was stuck to the longitudinally midpoint of each cuboid to adepth of 15 mm to form a conductor for electric energizing, and thenintegrally sintered to form porous sintered compact electrodes.

The production conditions and measurement results of the porous sinteredcompacts are shown in Table 5. As is apparent from the results, byadding the titanium suboxide to the raw material, a porous sinteredcompact having a sufficiently high hardness and the porosity, specificsurface area and pore distribution regulated by the present inventioncan be obtained at a further lower sintering temperature. TABLE 5Average Grain Size of Addition Titanium Amount Sintering Dioxide of Tem-Specific Pore Volume Ratio (%) Sintered Raw Titanium Molding pera-Surface Smaller Larger Compact Material Suboxide Pressure At- ture TimePorosity Area than 0.3-100 than Hardness No. (μm) (mass %) (MPa)mosphere (° C.) (hr) (%) (m²/cm³) 0.3 μm μm 100 μm (HV) Remarks 17 0.6*0 15 Open Air *900 11  *69  2.2 7.5 92.0 0.5 *55 Comparative Example 180.6 0.5 20 Open Air 900 1 45 2.2 10.1 84.8 5.1 62 Inventive Example 190.6 1.0 70 Open Air 1000 3 40 1.5 17.9 79.8 2.3 100 Inventive Example 200.6 1.0 20 Open Air 1100 4 25 0.5 22.3 75.6 2.1 150 Inventive Example 210.6 0 *120  Open Air 1200 6 *5 *0.0006 — *— — 320 Comparative Example 220.3 *0 20 Open Air *900 4 *66  4.5 27.7 60.9 11.4 *59 ComparativeExample 23 0.3 0.5 20 Open Air 900 1 55 4.0 22.4 70.1 7.5 80 InventiveExample 24 0.3 1.0 20 Open Air 1000 3 50 3.8 18.2 74.8 7.0 120 InventiveExample 25 0.3 1.0 20 Open Air 1100 4 45 2.2 28.5 65.2 6.3 150 InventiveExample 26 0.3 5.0 20 Ar 1400 5 35 2.0 49.9 50.0 0.1 130 InventiveExample 27 200 0.5 50 Open Air 900 1 42 0.9 0.1 35.5 64.6 75 InventiveExample 28 200 1.0 70 Open Air 1200 6 34 0.4 0.5 85.0 14.5 250 InventiveExample*denotes a value out of the range defined by the present invention.—: Incapable of measurement

Using the sintered compacts formed into the electrode shape,electrolysis was carried out for 10 hours by using a molten salt cellcomposed of CaCl₂ alone or in combination with NaCl, MgCl₂, CaF₂ or thelike, and a graphite electrode as the anode with an electrolyticpotential of 3.0V After the electrolysis, each electrode shape wasobserved, and the generation efficiency and average generation rate weredetermined based on the analysis of metallic Ti. These results arecollectively shown in Table 6. TABLE 6 Electrolytic Generation ofComposition of Condition Titanium Sintered Molten Salt Cell Cell CurrentGeneration Average ※ Test Compact (mass %) Temperature DensityEfficiency Rate Shape of No. No. CaCl₂ NaCl MgCl₂ CaF₂ (° C.) (A/cm²)(%) (kg/[hm²] Electrode Remarks D01 *17 100 0 0 0 850 0.50 5 0.14 XComparative Example D02 *17 50 20 20 0 900 0.80 1 0.04 X ComparativeExample D03 18 100 0 0 0 850 0.50 85 2.30 ◯ Inventive Example D04 18 5020 20 0 850 0.50 85 2.30 ◯ Inventive Example D05 18 40 30 30 10 850 0.5085 2.30 ◯ Inventive Example D06 19 100 0 0 0 850 0.50 80 2.16 ◯Inventive Example D07 20 100 0 0 0 850 0.20 75 0.81 ◯ Inventive ExampleD08 *21 100 0 0 0 850 0.10 5 0.03 ◯ Comparative Example D09 *22 50 0 0 0850 0.50 5 0.14 X Comparative Example D10 *22 40 20 20 0 980 0.50 7 0.19X Comparative Example D11 *22 100 30 30 10 700 0.50 2 0.05 X ComparativeExample D12 23 100 0 0 0 850 0.80 75 3.25 ◯ Inventive Example D13 24 1000 0 0 850 0.50 85 2.30 ◯ Inventive Example D14 25 100 0 0 0 850 1.50 705.68 ◯ Inventive Example D15 26 Inventive Example D16 27 100 0 0 0 8500.50 75 2.08 ◯ Inventive Example D17 28 100 0 0 0 850 0.50 70 1.89 ◯Inventive Example*denotes a value out of the range defined by the present invention.※ Evaluation of electrode shape◯: Maintaining the original shapeX: Collapsed

As is apparent from the results of Table 6, when the hardness isincreased by adding the titanium suboxide, the resultant porous sinteredcompact having the porosity, specific surface area and pore distributionspecified by the present invention is also an electrolytic raw materialexcellent in generation efficiency and average generation rate andsufficiently reducible without causing electrode collapse.

INDUSTRIAL APPLICABILITY

Using the porous sintered compact of oxide titanium of the presentinvention as an electrolytic raw material in the process whereintitanium oxide is reduced to metallic titanium by the electrolysis withan electrolyte composed of a molten salt enables efficiently obtainingmetallic titanium. The electrolytic process using a molten salt isattracting attention as a process capable of directly obtaining metallictitanium from titanium oxide with lower cost than in conventionalprocesses, and the employment of the above porous sintered compact wouldpromote its realization remarkably.

1. A porous sintered compact of titanium oxide for production ofmetallic titanium through direct electrolytic process, in which it has aporosity of 20 to 65% and a hardness of 60 (HV) or higher.
 2. A poroussintered compact of titanium oxide for production of metallic titaniumthrough direct electrolytic process, in which it has a porosity of 20 to65%, a specific surface area of 0.1 to 5.0 m²/cm³, and a volume ratio ofpores with 0.3 to 100 μm diameter to be 10% or higher to the total porevolume.
 3. A porous sintered compact of titanium oxide for production ofmetallic titanium through direct electrolytic process, in which it has aporosity of 20 to 65%, a hardness of 60 (HV) or higher, a specificsurface area of 0.1 to 5.0 m²/cm³, and a volume ratio of pores with 0.3to 100 μm diameter to be 10% or higher to the total pore volume.
 4. Aprocess for producing a porous sintered compact of titanium oxideaccording to any one of claims 1 to 3, comprising using a titanium oxidepowder having a grain size of 0.2 to 2000 μm, molding it into a requiredshape with pressurization in a range of 9.8 to 78.5 MPa, and sinteringit at 1100 to 1500° C. for 0.5 to 10 hours.
 5. A process for producing aporous sintered compact of titanium oxide according to any one of claims1 to 3, comprising adding and mixing 0.1 to 40%, based on mass, of atitanium suboxide powder to a titanium oxide powder followed by moldinginto a required shape, and sintering the resulting compact at 900 to1400° C. for 0.5 to 10 hours.
 6. A process for producing a poroussintered compact of titanium oxide according to any one of claims 1 to3, comprising using a titanium oxide powder having a grain size of 0.2to 2000 μm, adding and mixing 0.1 to 40%, based on mass, of a titaniumsuboxide powder thereto, molding into a required shape withpressurization in a range of 9.8 to 78.5 MPa, and sintering at 900 to1400° C. for 0.5 to 10 hours.
 7. A process for producing metallictitanium, comprising using a porous sintered compact of titanium oxideaccording to any one of claims 1 to 3, arranging it adjacently to aconductor or closely adhered around the conductor to constitute acathode, dipping it in a molten salt electrolyte of 800 to 1050° C.containing 40 mass % or more of calcium chloride, and reducing it byelectric energization.